The slip rate of a vehicle refers to the proportion of a sliding component in wheel movement and is represented as δ. The calculation formula is δ=(v−wr)/v×100%, wherein v represents the running speed of the vehicle on the ground (m/s); r represents the wheel rolling radius (m); and w represents the wheel angular speed (rad/s). As an important parameter, the slip rate is widely applied to vehicle control systems, and the slip rate needs to be kept within the range of 15%-20% in order to ensure the optimal braking effect, for example. As the slip rate is used to some extent in many cases, it is also very important to accurately measure the slip rate.
Currently, there are two types of commonly used slip rate detection methods for electric vehicles. In the first type of methods, the slip rate when the vehicle is operating is obtained by detecting the speeds of driving wheels and driven wheels of the vehicle, and the principle is replacing the wheel linear speed with the driving wheel speed, and replacing the vehicle speed with the driven wheel speed. Such a method is easy to implement, and currently adopted in most slip rate detection. However, the method needs mounting high-precision velocity sensors or acceleration sensors on the driving wheels and the driven wheels, and with the mounting of the sensors, not only can the system performance be reduced, but also as such a sensor usually uses an encoder for the implementation by counting impulse signals, not only does relay occur, but also the accuracy is affected and restricted by the resolution of the encoder. In addition, the method is only suitable for two-wheel drive vehicles, and can not be used for four-wheel drive vehicles due to lack of driven wheels, thus having limitation. For this reason, China patent document CN100480664C discloses a testing method for motion parameters of an all-wheel drive electric vehicle comprising the steps of: firstly, calculating the total ground longitudinal force and the full vehicle acceleration using wheel speed signals of the wheels and torque signals of the drive motors, and expressing the full vehicle speed using the rotational speed as well as the skid ratio and the slip rate of non-steering wheels; then substituting the above results into a differential equation of the slip rate of the non-steering wheels; and finally calculating the slip rate of the non-steering wheels by integration, and reversely deriving the full vehicle speed, and then calculating the slip rate of steering wheels. However, the method uses external drive torque and the wheel speed, and needs mounting a torque sensor in practical use, thus still having the problem about delay and accuracy resulting from the sensor; and the technical solution is only directed to all-wheel drive electric vehicles, and can not be applied to two-wheel drive vehicles, thus having limitation in use.
The other type of methods are slip rate calculation methods that do not need mounting a vehicle speed sensor; for example, a method for estimating the slip rate using the characteristic that ground change results in changes of other physical quantities is given in a dissertation titled Advanced Estimation Techniques of Road Surface Condition and Their Experimental Evaluation using Test Electric Vehicle ‘UOT March I and II” written by Kimihisa and Yoichi Hori; however, in the method, the system transfer characteristic from the rotational speed to the drive force is analyzed by adopting the Fourier transform, and then the change of road surface condition is analyzed; and as the spectrum analysis is complex in calculation and consumes relatively long time, the method is poor in real-time property, and relative difficult to implement. Patent US2009/0210128A1 discloses a method of obtaining the slip rate by resolving a state equation of slip rate, which is, estimating the slip rate from the wheel drive torque and rotational speed; and the method, however, needs additionally mounting a torque sensor, which is relatively high in cost, and the accuracy and reliability can also be affected.
In summary, all the slip rate calculation methods of the prior arts have the problem of being poor in real-time property.